1. Field of the Invention
The present invention relates to a dispersion compensating apparatus and a dispersion compensation control method for an optical transmission system.
In recent years, research and development of 40 Gbit/s optical transmission systems has actively advanced. One of the problems with realization of 40 Gbit/s optical transmission systems is that the chromatic dispersion tolerance of 40 Gbit/s optical signals is as small as 1/16 of that of 10 Gbit/s optical signals. The chromatic dispersion tolerance of 40 Gbit/s optical signals is, for example, approximately 70 ps/nm for nonreturn-to-zero (NRZ) modulated optical signals, and approximately 200 ps/nm for return to zero-differential quadrature phase shift keying (RZ-DQPSK) modulated optical signals.
Therefore, the effect of a change in the degree of chromatic dispersion caused by temperature changes of the transmission line and the dispersion compensating fiber (DCF) during the optical transmission system operation is too large to be ignored. To cope with fluctuations in the degree of chromatic dispersion during operation of the optical transmission system, automatic dispersion compensation using a variable dispersion compensating unit is required.
To realize automatic dispersion compensation by a variable dispersion compensating unit, a method of monitoring the bit error rate (BER) and controlling the compensation amount of the variable dispersion compensating unit to minimize the BER has been proposed such as the method described in Japanese Patent Application Laid-Open Publication No. 2002-208892. To monitor the BER, a method of monitoring the error correction rate in the forward error correction (FEC) can be used.
2. Description of the Related Art
In the method described above, not only the BER due to the effect of the chromatic dispersion but also the BERs due to factors other than chromatic dispersion are monitored without distinction. Therefore, even if the compensation amount by the variable dispersion compensating unit is controlled such that the BER is minimized, the amount of compensation by the variable dispersion compensating unit may not necessarily be the optimal compensation amount corresponding to the chromatic dispersion of the transmission fiber.
In particular, polarization fluctuations, an error factor other than the chromatic dispersion, drastically changes with a change in the state of the transmission line, such as a fiber contact. Therefore, if the compensation amount by the variable dispersion compensating unit is controlled by the method mentioned above without taking into account the BER due to factors resulting from polarization fluctuations, the compensation amount by the variable dispersion compensating unit is not necessarily the optimal amount.
Factors resulting from polarization fluctuations include, for example, polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent gain (PDG) caused by the transmission line and components such as an optical amplifier.
The present invention solves the problem and aims to provide a dispersion compensating apparatus and a dispersion compensation control method for optimally controlling the amount of compensation by the variable dispersion compensating unit, even when the bit errors due to factors other than chromatic dispersion occur.